The present invention relates generally to imaging of an object by electromagnetic radiation, especially to digital mammography.
1. Field of the Invention
More precisely, the invention relates to a digital imaging method in which the radiation that has passed through the object to be imaged is detected on at least one sensor, which contains one or more preferably elongated sensor modules, wherein the said sensor module contains one or more pixel columns which receive image data, in which method the object to be imaged is arranged essentially motionless and is scanned across with a beam which originates from a radiation source, the focus of which being essentially motionless in space, the beam being limited to be narrower than the object to be imaged and adapted essentially to the active surface of the sensor, and in which method the sensor is moved in synch with the scanning movement of the beam while at the same time the said active surface is kept essentially at right angles to the beam on the plane formed by the scanning movement of the beam.
The invention also relates to a digital imaging apparatus, which includes a radiation source, a sensor arrangement for detecting radiation, which arrangement contains one or more sensors formed of one or more preferably elongated sensor modules, which sensor module contains one or more pixel columns which receive image data, means for positioning the object to be imaged, the said means being situated within the area between the radiation source and the sensor arrangement, means for limiting the beam from the radiation source essentially according to the active sensor surface of the said sensor arrangement, means to move the beam across the object being positioned to be imaged and means to move the said at least one sensor belonging to the sensor arrangement in synch with the said scanning movement of the beam and to keep the said active sensor surface essentially at right angles to the beam on the plane formed by the scanning movement.
2. Description of Prior Art
In medical x-ray technique digital imaging provides certain advantages compared to the use of film. For example, fewer retakes are needed when a separate photograph developing stage is left out and when a major portion of the “failed” images even may be programmatically adjusted into a form still diagnostically applicable. On the other hand, the radiation dose the patient is exposed to decreases due to the semiconductor sensors being more sensitive than analogous films. While health care and hospital systems move more and more to digital technique in general and thus also to handling the x-ray images and patient information etc. in digital form, there additionally arises new possibilities and advantages related, among other things, to viewing, handling, storing and remote observing of the images having been taken and stored in digital form.
Semiconductor sensors for digital imaging purposes are typically radiation sensitive surfaces formed of small picture elements, or pixels, the extreme case of such surfaces being a line detector with a single line. The electromagnetic radiation, such as light, infra-red or x-ray radiation, which has been absorbed to the area of the pixels forms an electric charge corresponding the quantity and energy of the radiation quanta. So, when the electric charge is formed as a function of time, i.e. when during the ‘exposure time’ a pixel integrates the electric charge formed within its area, the level of the pixel signal may be adjusted in principle by altering the integration time. However, varying of the integration time does not affect sensitivity of the sensor.
Digital imaging may be implemented as full field imaging where a sensor according to (at least) the dimensions of the object is used, or as scanning imaging where a narrow sensor is used. In view of a practical imaging process, full field imaging corresponds to the traditional imaging onto a film of the size of the whole imaging area. A clear disadvantage of this technology is the need for sensors that are large in area and thus very expensive, and on the other hand the need to take into account the secondary radiation scattering from the object being imaged, which requires e.g. arranging complex mechanical grid structures in front of the detector. Because of their operational principle, the grid structures also even double the radiation dose needed for the imaging.
Narrow sensor is typically used in scanning technique, which requires some mechanics for support. However, such a solution is considerably more economical in total costs than solutions based on a full field sensor, especially due to its smaller sensor area. In scanning imaging also the grid structure may be left out.
Due to the high resolution, i.e. small pixel size needed in mammography, scanning imaging requires in practise use of a sensor of several pixels wide and a so called TDI method (Time Delay Integration) in order to achieve a signal that would be adequate for detecting the radiation by radiation-production of a practical magnitude. Although there are some other possibilities, TDI imaging is usually implemented by CCD sensor technique (Charge Coupled Device).
In U.S. Pat. No. 5,526,394 there has been presented a prior art digital scanning imaging solution, according to which scanning movement of the beam and the corresponding movement of the sensor arrangement is implemented in a mammography apparatus in mechanical connection with each other with the help of a pendulum in such a way that a collimation element limiting the beam and the sensor arrangement move along a concentric curved path. In the apparatus in question also the compression paddles, which position the tissue to be imaged, have been arranged curved according to the trajectory of the sensor arrangement. The focus of the swinging movement in the apparatus has been arranged to be situated on the level of the focus of the radiation source.
Although it is in principle mechanically simple to keep the sensor arrangement at right angles to the beam according to the solution of the above-mentioned publication, use of it also causes certain problems. For example, as it has been customary in mammography to position and compress the object to be imaged motionless between plane-like compression paddles, the curved compression surfaces are difficult to approve for many people to begin with. Practical problems may also occur, especially when small breasts are being positioned between wide curved surfaces. Additionally, such a way of positioning the object causes the imaging geometry becoming different compared to the traditional one, which geometry is further affected differently by the thickness of the tissue to be imaged than in the traditional solution. Furthermore, when using curved surfaces, typical special imaging modes used in mammography, such as enlargement, spot and stereotaxic imaging must be implemented in a completely new way, in which case they require specific solutions of their own, and all the traditional imaging modes are not even realizable in connection with such a solution—at least not without completely new special arrangements.
One of the main objects of the present invention is to promote development in digital mammography in such a way that even when scanning imaging is used, from the user's point of view both imaging apparatus and the image to be formed essentially correspond to the traditional film-based full field imaging, i.e. that in case so desired, the invention may be implemented “in a way which is (in principle) invisible to the user of the mammography apparatus”. Thus, an additional object of the invention is to enable modifying the existing film-based devices to digital ones with as small changes and costs as possible.
The essential features of the invention are expressed more precisely in the attached claims. These features include that when during the imaging scan the sensor surface is kept continuously at right angles to the beam on a plane formed by its scanning movement, alike according to prior art, the sensor is not moved along a curved path in direction of the scanning movement but essentially along a linear path.